Presently there are many different types of attitude sensors employed by spacecraft to determine attitude. Star trackers, sun sensors, earth sensors and gyroscopes are common. The required accuracy of such attitude sensors is usually determined by the required pointing accuracy of the payload. Laser crosslink payloads typically require highly accurate pointing accuracy. The cost of such attitude sensors generally increases with the sensor accuracy. Thus, in a constellation of satellites, use of high performance dedicated attitude sensors on each satellite represents a significant portion of the overall constellation expense.
Prior art techniques for satellites with optical crosslink payloads have typically assumed that the crosslinks are initially acquired and closed by equipping the satellite with attitude sensors of high enough accuracy such that the uncertainty in the pointing angles of the crosslink to the target satellite (the "field of uncertainty", or FOU) is smaller than the acquisition field of view (FOV) of the crosslink receive telescope or the FOV of the crosslink acquisition transmit beam. Calculated acquisition times, or crosslink closure times, for such systems are typically less than three minutes, and frequently on the order of a few seconds.
These high performance features of prior art systems are not otherwise needed during the mission since, once the crosslinks are closed, the crosslinks themselves provide extremely accurate attitude data that can be used to maintain the crosslinks and perform crosslink handoffs as needed.